Experimental autoimmune encephalomyelitis (EAE) is an animal model that reproduces many of the clinical and pathological features of multiple sclerosis (MS). The development and the progression of EAE, like other autoimmune diseases, results from the pathogenicity of effector cells and the negative regulation imposed by regulatory T cells (Tregs). Since Tregs express the Fox-P3 transcription factor, we have generated a Foxp3- EGFP knock-in mouse strain in order to track CD4+ CD25+ Fox-P3+ Tregs by GFP-expression. The Fox-P3- EGFP reporter mice readily develop EAE following immunization with myelin oligodendrocyte glycoprotein (MOG). With the help of an I-Ab/MOG35-55 tetramer, we are able to track the development and effector functions of MOG specific effector and Fox-P3+ regulatory T cells. Of all the encephalitogenic effector T cells, in recent years IL-17 producing Th17 cells have gained a lot of attention and are implicated in mediating tissue inflammation including EAE. We have found that IL-6, an acute phase protein, not only suppresses the generation of Fox-P3+ Treg cells induced by TGF-, but in turn induces Th17 effector T cells. Based on this observation, we hypothesize that there is a reciprocal relationship between pathogenic Th17 cells and protective Fox-P3+ Treg cells, and cytokines like IL-6 produced by the innate immune system dictate the balance between pathogenic/regulatory T cells and the development of autoimmunity. In addition to IL-6, we have now observed that IL-4 can also suppress induction of Fox-P3 and together with TGF- can induce a unique population of T cells that produces IL-9 and IL-10. These IL-9/IL-10 producing Th9 cells are not regulatory T cells but effector T cells that induce colitis and peripheral neuritis upon adoptive transfer into immunodeficient mice, however, what is their role in the induction of EAE has not been evaluated. We propose to: 1) Test the role of IL-6 in promoting tissue inflammation and in regulating the balance between pathogenic Th17 and Treg cells; 2) Study the role of Th9 cells in the induction and regulation of EAE; and 3) Study the role of IL-23, a cytokine that expands and stabilizes Th17 cells, in generating pathogenic effector T cells and induction of tissue inflammation in the CNS. For studying the role of IL-23 in EAE, we have generated a novel IL-23R.GFP knock-in mouse strain, which in a heterozygous state becomes a reporter for IL-23 receptor expression and in the homozygous state becomes a knock-out line. With the availability of reporter mouse strains for Fox-P3+ and IL-23R+ cells, we are in a unique position to study how inflammatory cytokines particularly IL-6 regulate the balance between pathogenic and regulatory T cells and promote tissue inflammation and EAE.